1. Field of the Invention
This invention relates to a phosphor, and more particularly to a rare earth element activated divalent metal fluorohalide phosphor.
2. Description of the Prior Art
As one kind of fluorohalide phosphors, there has been known in the art a rare earth element activated divalent metal fluorohalide phosphor in which a fluorohalide compound of divalent metal as alkaline earth metal constitutes the host material and a rare earth element constitutes the activator. For example, Japanese Patent Publication No. 28591/1976 discloses a divalent europium activated divalent metal fluorohalide phosphor represented by the formula EQU (Ba.sub.1-x-y-p,Sr.sub.x,Ca.sub.y,Eu.sup.2+ p)F(Cl.sub.1-a-b,Br.sub.a,I.sub.b)
wherein x, y, p, a and b are numbers satisfying the conditions of x+y+p.ltoreq.1, y.ltoreq.0.20, 0.001.ltoreq.p.ltoreq.0.20 and a+b.ltoreq.1. The phosphor emits light in the region of near ultraviolet rays having an emission peak in the neighbourhood of 390 nm under excitation of X-rays, ultraviolet rays, cathode rays, or the like. Since the phosphor has a high X-ray absorption efficiency and the emission spectrum thereof coincides with the spectral sensitivity of an X-ray film, the phosphor is practically used as a phosphor for a radiographic intensifying screen.
Such a rare earth element activated divalent metal fluorohalide phosphor as disclosed in the above-mentioned Japanese Patent Publication No. 28591/1976 can be obtained by firing a raw material mixture thereof at a temperature within the range of 600.degree. to 1000.degree. C. for a proper period of time. However, the phosphor is apt to sinter during the firing step. Accordingly, in the preparation of the phosphor, pulverization and classification of the phosphor must be conducted after the firing step, which results in not only complication of the preparation but also remarkable lowering of the yield of the phosphor. Further, it is well known in the art of phosphor preparation that the sintering of a phosphor generally affects the emission properties thereof, particularly the emission efficiency. Also in the rare earth element activated divalent metal fluorohalide phosphor, it seems that the sintering phenomenon during the firing step effects the emission efficiency of the phosphor.
As described in U.S. Pat. No. 4,080,306, the sintering phenomenon of the rare earth element activated divalent metal fluorohalide phosphor can be mitigated by adding a flux such as KCl, NaCl, LiCl, BaCl.sub.2 and MgCl.sub.2 to the raw material mixture of the phosphor. However, when the flux is used, the phosphor must be washed with water to remove the flux therefrom after the firing step. Therefore, a water washing step is needed instead of the pulverizing and classifying steps. Accordingly, although the period of time necessary for processing the phosphor can be shortened to some extent by using the flux, it is difficult to say that the preparation of the phosphor is simplified thereby. Further, since the rare earth element activated divalent metal fluorohalide has a relatively high solubility in water, the phosphor dissolves in a water together with the flux during the water washing step and therefore, the yield of the phosphor lowers. From this viewpoint also, the use of the flux is not desirable.
Further, since the rare earth element activated divalent metal fluorohalide phosphor is hygroscopic, the fluidity of the phosphor as a powder is low and the phosphor has a defect that it is apt to agglomerate. Accordingly, in the practical use of the phosphor, the handling thereof is frequently troublesome due to the agglomeration.
Furthermore, the afterglow property of the rare each element activated divalent metal fluorohalide phosphor is not satisfactory from the viewpoint of the practical use thereof. That is, the decay of the afterglow of the phosphor is relatively low.
As described above, the rare earth element activated divalent metal fluorohalide is nowadays popularly used in the radiographic intensifying screens. Therefore, the above-mentioned defects of the phosphor are serious problems in both the cost of preparation of the phosphor and the properties thereof, and it is desired to solve the problems.